1. Field of the Invention
The field of the invention is electrical power generation, particularly an apparatus for starting an engine driving a permanent magnet generator. The invention also relates to a start inverter apparatus feeding multiphase power to a permanent magnet generator to drive the generator as a motor and thereby spin an engine connected to the generator to a speed sufficient to permit the engine to start.
2. Description of Related Art
Utilizing a generator as a motor is a conventional technique well known to those of skill in the art. Furthermore, driving a generator as a motor to spin an engine connected to the generator to permit the engine to be started is also conventional.
There are a variety of motor control devices that are utilized to start electric motors. Indeed, such motor control devices have been reduced to a single chip. One example is the Microlinear ML4425/ML4426 motor controlling chip that is typically utilized to start a brushless DC motor. FIG. 8 illustrates the start-up sequence generated by this chip which includes three phases, the reset/align phase, the open-loop phase and the closed-loop running phase. This Microlinear chip has the advantage of not requiring active rotor position sensing. Instead, the chip utilizes a reset/align phase in which the rotor is forced to align to a known position.
As shown in the timing diagram of FIG. 8, this reset/align phase is triggered by a low reset (bar) signal. At the end of the reset/align phase, the reset (bar) signal changes sign thereby initiating the open loop phase.
With the position of the rotor now known, the open loop commutation phase can then successfully utilize a commutation sequence to generate a rotating field that drives the rotor. The commutation states that comprise this commutation sequence are shown in FIG. 9. This Figure also illustrates all of the switch closure On/Off states that control commutation. Because such commutation is generally known, no further description is necessary.
However, reference is made to FIG. 10 which further illustrates the commutation sequence in a wye connected motor. The commutation states illustrated in FIGS. 9 and 10 control the gate drive signals of the motor thereby causing a current to flow in the motor windings as shown in FIG. 10. As the states step from A to F (FIG. 9) a rotating magnetic field is developed which rotates the permanent magnets on the rotor shaft.
The 6-step sequence (A-F) in the open loop phase is repeated until enough back EMF is generated to allow a phase-lock loop circuit to lock into and control the commutation during the closed loop operation. This closed loop operation or phase is initiated by an enable E/A signal that changes sign as further illustrated in FIG. 8.
The reset (bar) and enable E/A signals are generated by the motor controller chip. The time relationship of these signals may be adjusted by setting a capacitor value based on motor dynamics.
Although such conventional motor controlling-chips would be quite useful in driving a permanent magnet generator as a motor to thereby spin an engine to a speed sufficient that the engine can be started, there are several obstacles that prevent such an adaptation. First of all, such conventional motor controlling chips do not produce enough current to drive a typical permanent magnet generator.
Furthermore, the position of the rotor, the DC voltage, and high static shaft friction can contribute to unsuccessful motor starting. In other words, there is typically a shaft that is driven by the engine to spin the permanent magnet generator. Such shafts have a high coefficient of friction particularly when they are first rotated due to the difference between the static coefficient of friction and the dynamic coefficient of friction.
Still further, such shafts may also include an air bearing which requires a relatively high RPM until bearing liftoff occurs. Once bearing liftoff does occur, then such air bearings are nearly frictionless. However, until such liftoff occurs there is a very high coefficient of friction that must be overcome by the permanent magnet generator being driven as a motor in order to successfully spin the engine to a speed sufficient to permit starting. Merely utilizing a conventional motor controller chip to drive a permanent magnet generator as a motor would fail because the typical open loop ramp rate does not allow the RCO/VCO voltage to raise high enough to generate sufficient back EMF for the phase-lock loop circuit to transition to a closed loop operation.
Even if the open loop ramp rate is increased to realize a higher voltage, the initial VCO frequency is too fast to commutate the rotor. Furthermore, if the ramp time is extended to realize a higher voltage, the rotor does not accelerate rapidly enough and will not follow the ramp. Thus, commutation with the rotor is lost and motor starting fails. All of these problems which have been recognized by the inventors of this application prevent commercially available motor controller chips from being applied to engine starting via a permanent magnet generator being driven as a motor.
Furthermore, conventional circuits that drive a permanent magnet generator as a motor utilize active rotor position sensors in order to control the commutation of the generator to drive it as a motor and thereby spin the engine. Such active rotor position sensors provide a high degree of control that is typically necessary to drive the permanent magnet generator as a motor.
However, such active rotor position sensors are subjected to an extremely harsh environment including high temperatures and high rotor speeds. Furthermore, such sensors must be precisely aligned with the magnets which necessarily increases the cost. The harsh environment and susceptibility to misalignment all contribute to low reliability.
Therefore, there is a need for a motor controlling circuit that can withstand the harsh environment presented by a permanent magnet generator with high reliability and low cost. This need is particularly acute because of the deregulation of the electric utility industry which now demands smallscale, low cost and highly reliable electrical power generation systems.